This invention relates to photodetectors and more particularly to infrared-sensitive photodiodes.
It is well established that single crystal films of lead chalcogenides, lead tin chalcogenides, and lead cadmium chalcogenides can be epitaxially grown on heated alkali metal halide and alkaline earth halide substrates by vacuum evaporation. The chalcogenides used include the sulfides, selenides, tellurides, and mixtures thereof. The substrates are single crystals of infrared transparent alkali metal halides and alkaline earth halides. Examples include barium fluoride, strontium fluoride, calcium fluoride, lithium fluoride, sodium chloride, potassium chloride, etc.
It is also well known that the vacuum deposition of a metal contact of certain materials such as lead or indium, on the surface of an epitaxial layer of a lead chalcogenide, lead tin chalcogenide, or lead cadmium chalcogenide creates a non-Ohmic Schottky barrier at the point of contact, resulting in an infrared sensitive photodiode. Vacuum depositing a contact of certain other metals (e.g., Au, Ni, or Pt) at another point on the epitaxial layer provides the Ohmic contact necessary for measuring the photovoltage of the device.
Attention is called to U.S. Pat. No. 4,263,604, entitled "Graded Gap Semiconductor Detector," issued on Apr. 21, 1981 to James D. Jensen and Richard B. Schoolar wherein an extensive bibliography of articles and patents dealing with these Schottky barrier devices is listed in the background of the invention.
Alan C. Bouley, Harold R. Riedl, James D. Jensen, and Richard B. Schoolar in U.S. Pat. No. 4,442,446, entitled "Sensitized Epitaxial Infrared Detector," disclose that the presence of halide ions in the interface region between a non-Ohmic lead metal and an epitaxial layer of II-IV-VI semiconductor alloy material (i.e., lead chalcogenide, lead tin chalcogenide, or lead cadmium chalcogenide) increases the performance of these infrared detecting diodes. The number of rejects in the manufacturing process is also reduced. In their process Bouley et al. mix the lead halide salt in with the lead used to form the epitaxial layer of semiconductor material. As a result, the halide ions are distributed throughout the semiconductor layer.
Tak-Kin Chu and Alan C. Bouley in U.S. Pat. No. 4,406,050, entitled "Method for Fabricating Lead Halide Sensitized Infrared Photodiodes," disclose a method in which the epitaxial layer of semiconductor material is first deposited on an infrared transparent substrate in the conventional manner. Then a layer of lead halide (PbCl.sub.2, PbBr.sub.2, PbF.sub.2) is vapor deposited in air at atmospheric pressure on the surface of the semiconductor material prior to the vacuum deposition of the non-Ohmic Pb metal contact. The lead halide source and the epitaxial layer/substrate are kept at the same temperature in the range of from about 175.degree. C. to about 225.degree. C. This procedure produces even fewer rejects than the Bouley et al. process. Unfortunately, this PbCl.sub.2 deposition step requires at least 6 hours and preferably at least 8 hours. As a result, the rate of production is much lower than is desirable. Therefore, it would be desirable to achieve the same excellent results as the Chu et al. process but in a much shorter time.